The manufacture of integrated circuits involves laying down many layers of different materials each of which is formed into a different pattern using standard photolithographic techniques. In general, one wafer is processed at a time so that differences in the widths of lines of the actual photoresist patterns will vary somewhat from wafer to wafer. An example of this is illustrated in FIG. 1 which shows the CD (critical dimension or minimum width) for lines that were formed using the same reticle, or mask, on eight different wafers.
Lines 11 and 12 in FIG. 1 represent the upper and lower bounds for the CD to fall within the specified limits. In this example, wafers 2 and 6 had a CD that was too high while wafer 7 had a CD that was too low. Variations of this sort are the result of small, but unavoidable, changes that take place during exposure and subsequent processing of the photoresist. These include such factors as the numerical aperture and depth of focus of the optical reduction system as well as the times and temperatures used to develop the photoresist.
The most common approach to dealing with this in the prior art has been to simply strip off the photoresist whenever the CD is found to be out of spec. and then start again. This can be expensive and does not guarantee that the problem will not arise again.
A somewhat more sophisticated approach used in the prior art is to measure the wafers as in their patterns are generated, accepting those that are within spec and then using the out of spec data to adjust the full photolithographic process (numerical aperture, development parameters, etc.). A flow chart representation of this method is shown in FIG. 2. While this approach reduces the wafer to wafer variations in CD it is not sufficiently reliable to allow removal of the feedback loop once the system appears to have settled down. Additionally, deciding what changes are to be made to the photolithographic process, based on the measured CDs, can be quite complicated and difficult to implement.
Another problem associated with the photolithography of fine lines is that of edge roughness and foot formation. Inevitably the edges of lines formed in photoresist cannot be completely straight and a certain amount of ripple will appear. As lines get to be finer and finer this edge roughness begins to be a significant fraction of the actual line width. In FIG. 3a we show an example of foot formation in a line of photoresist. Line 30 is about 0.16 microns wide and about 0.38 microns high. Foot 31 can be seen extending about 0.015 microns outward at its base. The presence of the foot introduces a level of uncertainty into the width of any line that is subsequently etched using this line of photoresist as a mask. The prior art process discussed above provides no correction for either the roughness problem the line foot problem.
Also used in the prior art is a process whereby the width of the photoresist lines can be reduced by exposing them to a gas plasma discharge. This approach is satisfactory when the measured CD of the photoresist lines is too high (such as for wafers 2 and 6 in FIG. 1), but cannot be used when the CD is too low (such as for wafer 7 in FIG. 1). An example of a photoresist line whose foot has been removed through exposure to a gas discharge plasma is shown in FIG. 3b.
A routine search of the prior art was conducted but no references teaching the exact method of the present invention were found. Several references of interest were, however, seen. For example, Yang (U.S. Pat. No. 5,913,102) shows a system for controlling CD using a measurement parameter and a control parameter. Yang does not specifically discuss photoresist trimming processes related to CD of gates
Muller et al (U.S. Pat. No. 5,674,409) show a photoresist trimming process that uses ashing while Shinagawa et al. (U.S. Pat. No. 5,057,187) show a method of controlling such an ashing process.
Leung (U.S. Pat. No. 4,717,445) shows an etch bias monitoring technique while Bindell et al. (U.S. Pat. No. 5,804,460) show a method for measuring photoresist line widths.